1. Field of the Invention
The present invention relates to an apparatus for detachably fixing a sheet member onto a rotor such as a rotary drum, and to an image forming apparatus incorporating the former.
2. Description of the Related Art
Generally, a photosensitive printing plate (hereinafter, “printing plate”), comprising a sheet-like support (e.g., a thin aluminum plate) having disposed thereon a photosensitive layer, is used for printing. Printing plates having different vertical and horizontal dimensions are used depending upon the size of the item to be printed.
An image exposure apparatus that image-exposes the printing plate includes a device for winding the printing plate around a rotary drum and irradiating the printing plate with a light beam corresponding to image data while the printing plate is rotated integrally with the rotary drum, whereby the printing plate is scan-exposed.
When the printing plate is wound around the rotary drum, the printing plate is nipped and fixed to the rotary drum by holding devices (chucks). The holding devices clamp ends of the printing plate along the circumferential direction of the rotary drum to thereby fix the ends between the chucks and the peripheral surface of the rotary drum.
Namely, a holding device corresponding to one end (e.g., a leading edge in the direction in which the printing plate is wound) of the printing plate along the circumferential direction of the rotary drum is attached at a predetermined position on the rotary drum. After the printing plate has been wound around the rotary drum, a holding device corresponding to the other end (i.e., a trailing edge) of the printing plate is attached at a position according to the size of the printing plate.
The holding device for attaching the printing plate to the rotary drum at the position according to the size has a fixing barrel (stanchion) which can be inserted into an arbitrary position of an attachment groove that is formed along the peripheral direction in the peripheral surface of the rotary drum. The fixing barrel is freely movable relative to the attachment groove, whereby the ends of printing plates having various sizes can be nipped in suitable positions. The attachment groove for housing the fixing barrel has a transverse cross section that can be quite complicated.
However, this type of rotary drum necessarily holds considerably large, solid portions in which such grooves can be formed. This means that the rotary drum inevitably becomes large and heavy, which poses an obstacle to high-speed rotation of the rotary drum.